Replication competent vaccinia virus (W), the current vaccine for smallpox, can cause severe complications after vaccination, especially in immune suppressed individuals. We are seeking to develop attenuated strains of W that are replication competent, and thus will induce a strong immune response, without the complications associated with vaccination with the current vaccines. Thus, the Aim of this proposal is to further develop existing strains of vaccinia virus that contain mutations in a key innate immune evasion gene, the E3L IFN-resistance gene, as safe effective alternatives to the current and proposed smallpox vaccines. We have deleted the E3L gene from a tissue culture adapted variant of the NYCBH strain of W, ACAM2000. The relative immunogenicity, adjuvancy and safety of ACAM2000delE3L will be compared with that of ACAM2000 and Dryvax in mice (immunogenicity and safety). Efficacy will be tested against ectromelia, rabbitpox and monkeypox challenges in mice, rabbits and macaques, respectively, compared to Dryvax. The intent is to use this data to satisfy the FDA Animal Efficacy Rule, and pave the way for Phase l/ll Clinical Trials in healthy human volunteers.